Question about the ending of parasite

At the end of the film, it is shown that Kim goes hiding into the bunker. But I have some questions regarding this.

1. Given that two murders took place in the mansion, it can be assumed that there was a proper investigation of the whole incident. So, how come the police didn't find out the bunker? The basement was full of Kevin's blood, so it is very possible that a blood trail can be found up to the basement. It shouldn't be very hard for the police to find out about the bunker. But they never do.

2. At the end, it is shown that kevin and his mom are convicted of fraud and placed on probation. As they were convected, they must have been interrogated by the police. Up to that point, they didn't know that Kim hid in the bunker. So, they didn't have any reason to hide the actual things that happened on that night. Then, how come the police didn't find out about the bunker?

3. A possible explanation can be a scene shown after Kevin wakes up. He sees a very young police officer asking him some questions who doesn't look like a police officer. So this scene may mean that the investigation was not handled properly by the police. But, I don't see in any way that given the blood trail up to basement and possibility of interrogation of two convicted criminals (Kevin and his mom), the police were unable to know about the existence of a bunker in the house.

Can someone explain this?

TL;DR: How come the police didn't find out about the bunkers at the ending of 'Parasite'?